As a car moves, there occurs continuous changes in the relationship between direct waves from a broadcasting station and indirect waves reflected by mountains and buildings. When the relationship meets a certain condition, a multipath distortion occurs due to interference between the direct waves and indirect, reflected waves. The multipath distortion is one of serious problems of a car-use FM tuner.
The multipath distortion causes multipath noises to appear in audio outputs and significantly degrade the reproduced sound. There are two prior art countermeasures against the multipath distortion. One of them is the use of two or more receivers in a single receiver system such as in a space diversity receiver system having two or more antennas to select one of entered signals through the antennas which include the least multipath distortion. The other countermeasure is the use of a circuit for alleviating the noise (i.e., the use of a signal-strength(S) meter voltage for high cut or high blend control of the stereo outputs).
Alleviation of multipath noises is taught by Japanese Patent Preliminary Publication No. 212830/1982. This publication teaches detection of multipath noises from a signal-strength meter to subsequently reduce the noises in the following two methods.
(a) Automatic Switching between monoral and stereo receptions (high blend control):
A noise in the monoral reception is about 20 dB smaller than a noise in the stereo reception. When the antenna input level decreases, and a noise in the stereo output becomes relatively large, the stereo reception is changed to the monoral reception to decreases the noise in the demodulated output to the noise level of the monoral reception.
(b) High band attenuation (high cut control):
Among all bands noise components, high band noise components in particular bother human ears. Therefore, when the noise is large, it is reduced by attenuating high band components in the demodulated output.
The aforegoing two methods are significantly effective against multipath noises. More specifically, since multipath distortion includes a lot of higher-order and higher-harmonic components, a substantial part of the distortion can be reduced in high demodulation frequencies by an emphasis circuit (circuit for effecting the high cut control). The aforegoing Japanese publication fixes a threshold level corresponding to a single diode to prevent erroneous operation of a noise reduction circuit. When the modulation signal frequency changes, an FH modulation wave changes in amplitude due to a side-lobe expansion caused by a change of the FM modulation ratio. A multipath detecting circuit detects the change in the amplitude as a multipath noise and produces a detection signal regardless of absence of multipath noises. Therefore, a high cut and high blend control is erroneously effected accordingly. To avoid this error, the aforegoing Japanese publication employs a diode, and fixes a threshold level so as to displace the detected level of the multipath noise by an amount corresponding to the forward voltage of the single diode. However, the threshold level is limited to the diode and cannot be selected freely. Further, since the time constant for charge and discharge in the noise reduction circuit against generation of multipath noises cannot be fixed independently, a limitation is imposed to the output response of the circuit against noises. Therefore, reproduced sound is hardly improved in regions where multipath noises are often generated.
The prior art system includes a negative-phase amplifier to amplify ripple components included in an S meter output voltage. The amplifier signal is negative rectified into a negative voltage, and the negative voltage is mixed with a d.c. voltage in the S meter output voltage into a resulting voltage which is used as a control voltage for control of the high cut and high blend operation.
FIG. 5 shows a circuit diagram of a noise reduction circuit disclosed by the aforegoing Japanese publication, wherein reference numeral 1 designates an input terminal in which an S meter output voltage V.sub.S is entered, and reference numeral 2 denotes an output terminal where a high cut and high blend control voltage V.sub.O appears.
When a multipath disturbance occurs, negative ripple components are included in the S meter output voltage. Additionally, noises such as plug noises normally appear as positive ripple components. FIG. 6 shows waveforms of the S meter output voltage upon multipath disturbance at (a) and upon plug noises at (b).
In the prior art arrangement of FIG. 5, ripple components in the S meter output voltage are amplified through the negative-phase amplifier comprising a transistor Tr, etc., and the amplified signal is negative rectified by diodes D.sub.1 and D.sub.2 into a d.c. output voltage which is subsequently mixed with the d.c. components in the S meter output voltage into a resulting voltage used as the high-cut and high-blend control voltage V.sub.O. More specifically, when multipath noises are generated, ripple components caused by the multipath noises in the S meter output voltage are positively inverted and amplified by the negative-phase amplifier.
Further, ripple components caused by plug noises are negative rectified to negatively charge the capacitor C.sub.3. Actually, since a coupling capacitor C.sub.1 forms a differential circuit, positive ripple components appear in the base input waveform of the transistor Tr regardless of negative ripple components generated during multipath disturbance. Therefore, multipath noises are negative rectified, too. The aforegoing circuit is low in sensitivity to multipath noises, but high to plug noises.
The circuit of FIG. 5 operates as follows.
The capacitor C.sub.4 is charged to a potential V.sub.1 via a resistor R.sub.6. multipath or other noises are generated, the potential of the capacitor C.sub.3 decreases. When the potential becomes below the forward voltage V.sub.F of the diode D.sub.3, the capacitor C.sub.4 discharges so as to elevate the potential of the capacitor C.sub.3 through the diode D.sub.3 and resistor R.sub.5. Further, an electric current flows through the resistor R.sub.6 to charge the capacitor C.sub.4. The charging current to the capacitor C.sub.4 drops the potential V.sub.1. This is the theory of the circuit of FIG. 5. The charging time constant T.sub.1 is determined by C.sub.4 R.sub.6, and the discharging time constant T.sub.2 by C.sub.4 R.sub.5. Discontinuous impression of the sound quality due to multipath noises is significantly reduced by an appropriate value of the charging and discharging time constants. An ideal value of the charging time constant T.sub.1 is about 10 seconds. However, in the prior art system it is requested that capacitor C.sub.4 =1000 .mu.F and resistor R.sub.6 =10k.OMEGA., for example, to obtain the charging time constant of 10 seconds. Since the value of the resistor R.sub.6 is not increased so much, the capacitor must be significantly large. Therefore, the prior art system is disadvantageous in the manufacturing cost and space saving of the circuit. Further, a dead zone for inactivating the aforegoing circuit during small or modest multipath noises can be selected by appropriate selection of the value of the resistor R.sub.1 but is limited to an extent.
As described, the prior art control voltage generating circuit for activating the noise reduction circuit in an FM receiver is not so sensitive to multipath noises and rather sensitive to plug noises. More specifically, all the while that plug noises are generated, the high cut and high blend control voltage drops, and the control voltage generating circuit is activated to remove high frequency components from stereo demodulated signal outputs even if multipath noises are not generated. Additionally, since the charging time constant cannot be longer than about 10 seconds, the circuit is responsive too fast to a sudden change in the signal condition, which produces a discontinuous impression in the sound quality. Further, it has been difficult to provide a dead zone simply responsive to multipath noises above a predetermined level or to freely select the discharge amount of the capacitor upon reception of multipath noises of a predetermined level.